linux-kernel - Re: [PATCH v2 2/3] rust: Add support for deriving `AsBytes` and `FromBytes`

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Message-Id: <DF05VVOUA70W.30GJE9P4Z3OL9@nvidia.com>
Date: Wed, 17 Dec 2025 12:12:10 +0900
From: "Alexandre Courbot" <acourbot@...dia.com>
To: "Matthew Maurer" <mmaurer@...gle.com>, "Miguel Ojeda"
 <ojeda@...nel.org>, "Boqun Feng" <boqun.feng@...il.com>, "Gary Guo"
 <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, "Benno Lossin" <lossin@...nel.org>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>
Cc: <rust-for-linux@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] rust: Add support for deriving `AsBytes` and
 `FromBytes`

On Tue Dec 16, 2025 at 9:44 AM JST, Matthew Maurer wrote:
> This provides a derive macro for `AsBytes` and `FromBytes` for structs
> only. For both, it checks the respective trait on every underlying
> field. For `AsBytes`, it emits a const-time padding check that will fail
> the compilation if derived on a type with padding.
>
> Signed-off-by: Matthew Maurer <mmaurer@...gle.com>

I like this a lot. We have a bunch of unsafe impls in Nova that this
could help us get rid of.

Amazed that this even seems to work on tuple structs!

> ---
>  rust/macros/lib.rs       | 63 ++++++++++++++++++++++++++++++++++++++++++++++++
>  rust/macros/transmute.rs | 58 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 121 insertions(+)
>
> diff --git a/rust/macros/lib.rs b/rust/macros/lib.rs
> index b38002151871a33f6b4efea70be2deb6ddad38e2..d66397942529f67697f74a908e257cacc4201d84 100644
> --- a/rust/macros/lib.rs
> +++ b/rust/macros/lib.rs
> @@ -20,9 +20,14 @@
>  mod kunit;
>  mod module;
>  mod paste;
> +mod transmute;
>  mod vtable;
>  
>  use proc_macro::TokenStream;
> +use syn::{
> +    parse_macro_input,
> +    DeriveInput, //
> +};
>  
>  /// Declares a kernel module.
>  ///
> @@ -475,3 +480,61 @@ pub fn paste(input: TokenStream) -> TokenStream {
>  pub fn kunit_tests(attr: TokenStream, ts: TokenStream) -> TokenStream {
>      kunit::kunit_tests(attr, ts)
>  }
> +
> +/// Implements `FromBytes` for a struct.
> +///
> +/// It will fail compilation if the struct you are deriving on cannot be determined to implement
> +/// `FromBytes` safely. It may still fail for some types which would be safe to implement
> +/// `FromBytes` for, in which case you will need to write the implementation and justification
> +/// yourself.
> +///
> +/// Main reasons your type may be rejected:
> +/// * Not a `struct`
> +/// * One of the fields is not `FromBytes`
> +///
> +/// # Examples
> +///
> +/// ```
> +/// #[derive(FromBytes)]
> +/// #[repr(C)]
> +/// struct Foo {
> +///   x: u32,
> +///   y: u16,
> +///   z: u16,
> +/// }
> +/// ```

One thing I have noticed is that I could sucessfully derive `FromBytes`
on a struct that is not `repr(C)`... Is that something we want to
disallow?